1. Field of the Invention
The present invention relates to boundary acoustic wave devices using SH boundary acoustic waves, and particularly to a boundary acoustic wave device including an electrode disposed at the boundary between a LiTaO3 piezoelectric body and a dielectric body.
2. Description of the Related Art
A variety of surface acoustic wave devices have been used for cellular phone RF or IF filters, VCO resonators, television VIF filters, and other devices. Surface acoustic wave devices use surface acoustic waves propagating along a surface of a medium, such as Rayleigh waves or first leaky waves.
Since the surface acoustic waves propagate along a surface of a medium, they are sensitive to changes in the surface conditions. In order to protect the surface of the medium along which surface acoustic waves propagate, the surface acoustic wave element is hermetically enclosed in a package having a hollow portion in a region opposing the surface acoustic wave-propagating surface of the medium. The package having such a hollow portion inevitably increases the cost of the surface acoustic wave device. Also, since the package is much larger than the surface acoustic wave element, the size of the resulting surface acoustic wave device is increased.
In addition to the surface acoustic waves, acoustic waves include boundary acoustic waves propagating along the boundary between solids.
For example, “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and Ultrason., VOL. SU-25, No. 6, 1978 IEEE (Non-Patent Document 1) discloses a boundary acoustic wave device including an IDT provided on a 126°-rotated Y-plate X-propagating LiTaO3 substrate, and a SiO2 layer having a predetermined thickness disposed over the IDT and the LiTaO3 substrate. This device propagates SV+P boundary acoustic waves called Stoneley waves. Non-Patent Document 1 discloses that when the SiO2 layer has a thickness of 1.0λ (λ: wavelength of boundary acoustic waves), the electromechanical coupling coefficient is 2%.
Boundary acoustic waves propagate with their energy concentrated on the boundary between the solids. The bottom surface of the LiTaO3 substrate and the top surface of the SiO2 film, therefore, have very little energy, and the characteristics are not varied depending on the changes in the surface conditions of the substrate or the thin layer. Thus, the package having the hollow portion can be eliminated, and the size of the acoustic wave device can be reduced accordingly.
“Highly Piezoelectric Boundary Waves Propagating In Si/SiO2/LiNbO3 Structure” (26th EM Symposium, May 1997, pp. 53-58) (Non-Patent Document 2) discloses SH boundary waves propagating in a [001]-Si(110)/SiO2/Y-cut X-propagating LiNbO3 structure. This type of SH boundary waves features an electromechanical coupling coefficient K2 greater than that of the Stoneley waves. With the use of the SH boundary waves, similar to the use of Stoneley waves, the package having the hollow portion can be eliminated. In addition, since SH boundary waves have SH-type fluctuation, the strips defining an IDT reflector have a greater reflection coefficient as compared to those used for Stoneley waves. Therefore, the use of SH boundary waves for, for example, a resonator or a resonator filter facilitates reducing the size of the device and produces sharp characteristics.
Other related boundary acoustic wave devices are disclosed in “Highly Piezoelectric SH-type Boundary Waves” IEICE Technical Report Vol. 96, No. 249 (US96 45-53) pp. 21-26, 1996 (Non-Patent Document 3), and “Fabrication of Boundary Wave Device By Wafer Bonding Techniques” (in Japanese), Ozawa, Yamada, Omori, Hashimoto, and Yamaguchi, Piezoelectric Materials and Device Symposium, 2003, Piezoelectric Materials and Device Symposium Executive Committee, Feb. 27, 2003, pp. 59-60 (Non-Patent Document 4).
A boundary acoustic wave device must have an appropriate electromechanical coupling coefficient in accordance with the application and must have a low propagation loss, power flow angle, and temperature coefficient of frequency. In addition, low spurious responses are required in the vicinity of the main response.
Specifically, the loss of boundary acoustic waves accompanied with their propagation, that is, propagation loss adversely affects the insertion loss of the boundary acoustic wave filter, the resonant resistance, and the impedance ratio of the boundary acoustic wave resonator. The impedance ratio is the ratio of the impedance at the resonant frequency to the impedance at the anti-resonant frequency. Thus, it is desirable to reduce the propagation loss.
The power flow angle represents the difference in direction between the phase velocity of boundary waves and the group velocity of boundary wave energy. If the power flow angle is large, the IDT must be disposed at an angle according to the power flow angle. Accordingly, the electrode is complicated to design, and a loss resulting from the displacement of the angles is likely to occur.
The changes in operation frequency with temperature of a boundary wave device reduce the practicable pass band or stop band if the boundary wave device is a boundary wave filter. If the boundary wave device is a resonator, the changes in operation frequency with temperature cause abnormal oscillation in an oscillation circuit. Thus, it is desirable to reduce the TCF, which is a change in frequency per degree centigrade.
A low loss resonator filter may be constructed by, for example, disposing reflectors in the propagation direction outside the region provided with a transmitting and a receiving IDT for transmitting and receiving boundary waves. The bandwidth of the resonator filter depends on the electromechanical coupling coefficient when using the boundary waves. A higher electromechanical coupling coefficient k2 leads to a broadband filter, and a lower electromechanical coupling coefficient leads to a narrowband filter. Accordingly, boundary wave devices must use an appropriate electromechanical coupling coefficient k2 in accordance with their applications. For RF filters of cellular phones, the electromechanical coupling coefficient k2 should be at least about 3%, and preferably at least about 5%.
However, in the boundary acoustic wave device disclosed in the above-described Non-Patent Document 1 using Stoneley waves, the electromechanical coupling coefficient is as low as about 2%.